Decimate modifier is reducing faces, even though the set angle is smaller than the Angle Limit set
A test case:
From left to right: 90° then 30°.

If we go above 30° in the modifier the 30° angle is decimated:

Now we have a 105° angle:

which means the angle variation is 75° (180 - 105°) following the edge (or face) variation.
So if the modifier angle goes above 75°:

... the edge collapse again.
That means the modifier is &quot;cumulative&quot;: it will decimate all parts where the angle variation is below the given angle in the modifier.